Antimicrobials and Resistance Practice Questions
20 free Antimicrobials and Resistance practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
A patient treated with vancomycin for MRSA develops increasing MIC values over time. Which resistance mechanism is most likely?
- A Altered penicillin-binding proteins
- B Increased efflux pump activity
- C D-Ala-D-Ala replaced with D-Ala-D-Lac
- D Increased β-lactamase secretion
Correct answer: D-Ala-D-Ala replaced with D-Ala-D-Lac
Vancomycin resistance occurs when bacteria replace D-Ala-D-Ala with D-Ala-D-Lac, reducing drug binding affinity.
A patient on gentamicin develops ototoxicity. Which factor increases this risk?
- A Concomitant high-dose NSAID use
- B Concomitant loop diuretic use
- C Underlying hyperkalemia
- D Taking the drug with food
Correct answer: Concomitant loop diuretic use
Loop diuretics (e.g., furosemide) synergistically worsen aminoglycoside-induced ototoxicity.
A patient is prescribed imipenem. Why must cilastatin be co-administered?
- A Prevents drug-induced seizures
- B Enhances renal drug excretion
- C Inhibits renal dehydropeptidase I
- D Blocks β-lactamase binding sites
Correct answer: Inhibits renal dehydropeptidase I
Cilastatin inhibits renal dehydropeptidase I, preventing imipenem degradation and increasing its effectiveness.
A patient taking tetracycline is advised not to take it with milk. Which mechanism explains this interaction?
- A Enhanced hepatic drug metabolism
- B Insoluble calcium chelate formation
- C Increased renal drug clearance
- D Decreased hepatic drug uptake
Correct answer: Insoluble calcium chelate formation
Dairy products chelate tetracyclines, reducing absorption and antibiotic effectiveness.
A 25-year-old woman with UTI is prescribed TMP-SMX. Which pathway does this drug combination inhibit?
- A DNA gyrase and cell wall synthesis
- B Folate synthesis at two sequential steps
- C Protein synthesis at 50S and 30S
- D Bacterial peptidoglycan cross-linking
Correct answer: Folate synthesis at two sequential steps
TMP-SMX blocks folate synthesis at sequential steps, providing synergistic bactericidal action.
A patient treated for pneumonia develops Achilles tendon rupture. Which antibiotic is most likely responsible?
- A Azithromycin
- B Ciprofloxacin
- C Ceftriaxone
- D Clindamycin
Correct answer: Ciprofloxacin
Fluoroquinolones are associated with tendon rupture, especially in older adults.
A man develops C. difficile colitis after broad-spectrum antibiotic use. Which drug is most appropriate?
- A Oral metronidazole
- B IV vancomycin
- C Oral vancomycin
- D IV cefepime
Correct answer: Oral vancomycin
Oral vancomycin is first-line therapy because it stays in the gut lumen where C. difficile resides.
A patient on linezolid develops bone marrow suppression. What is this drug’s mechanism?
- A Binds 50S and prevents translocation
- B Blocks 70S initiation complex formation
- C Inhibits bacterial DNA gyrase
- D Inhibits dihydrofolate reductase
Correct answer: Blocks 70S initiation complex formation
Linezolid prevents formation of the 70S initiation complex, inhibiting protein synthesis.
A patient with syphilis is allergic to penicillin. Which alternative antibiotic is appropriate?
- A Azithromycin
- B Doxycycline
- C Ciprofloxacin
- D Clindamycin
Correct answer: Doxycycline
Doxycycline is an effective alternative for penicillin-allergic patients with syphilis.
A man receiving rifampin notices orange discoloration of body fluids. What additional effect should be monitored?
- A Bone marrow suppression
- B Inhibition of CYP450 enzymes
- C Induction of CYP450 enzymes
- D Severe nephrotoxicity
Correct answer: Induction of CYP450 enzymes
Rifampin is a strong CYP450 inducer and may decrease levels of many drugs.
A patient treated with isoniazid develops peripheral neuropathy. Supplementation with which vitamin prevents this?
- A Vitamin C
- B Vitamin B6
- C Vitamin B12
- D Vitamin E
Correct answer: Vitamin B6
Isoniazid causes pyridoxine (B6) deficiency, leading to neuropathy; supplementation prevents this effect.
A patient on aminoglycosides develops nephrotoxicity. What is the primary mechanism?
- A Damage to proximal tubule cells
- B Acute interstitial nephritis
- C Immune-mediated glomerulonephritis
- D Reduced renal blood flow
Correct answer: Damage to proximal tubule cells
Aminoglycosides accumulate in proximal tubule cells, causing acute tubular necrosis.
Why are aminoglycosides ineffective against anaerobic bacteria?
- A They cannot penetrate the cell wall
- B They require oxygen-dependent uptake
- C Anaerobes degrade the drug
- D Anaerobes have intrinsic ribosomal protection
Correct answer: They require oxygen-dependent uptake
Aminoglycosides require oxygen-dependent transport into bacterial cells, rendering them ineffective in anaerobic environments.
A patient with gonorrhea is treated with ceftriaxone. Which resistance mechanism could compromise treatment?
- A Altered porin proteins
- B Efflux pumps
- C β-lactamase production
- D Methylation of 23S rRNA
Correct answer: Altered porin proteins
Neisseria gonorrhoeae often develops resistance through porin mutations that prevent antibiotic entry.
A patient taking macrolides develops prolonged QT interval. What is the primary mechanism of macrolide action?
- A Blocks the 30S ribosomal subunit
- B Inhibits translocation at 50S subunit
- C Inhibits cell wall cross-linking
- D Inhibits dihydropteroate synthase
Correct answer: Inhibits translocation at 50S subunit
Macrolides bind the 50S subunit and prevent translocation during protein synthesis.
A patient with MRSA is started on daptomycin. Why should lung infections not be treated with this drug?
- A Poor penetration into lung tissue
- B Inactivation by pulmonary surfactant
- C Rapid development of resistance
- D Risk of pulmonary fibrosis
Correct answer: Inactivation by pulmonary surfactant
Daptomycin is bound and inactivated by surfactant, making it ineffective for pneumonia.
A 7-year-old child is being considered for doxycycline. Why should this drug be avoided?
- A Risk of aplastic anemia
- B Permanent tooth discoloration
- C High risk of peripheral neuropathy
- D Severe cardiotoxicity risk
Correct answer: Permanent tooth discoloration
Tetracyclines deposit in teeth and bones, causing discoloration and impairment in children.
A patient treated for TB develops orange urine and elevated LFTs. Which drug is responsible?
- A Isoniazid
- B Ethambutol
- C Rifampin
- D Streptomycin
Correct answer: Rifampin
Rifampin causes orange discoloration of secretions and is hepatotoxic.
A woman develops a rash after taking ampicillin for a sore throat. She is later diagnosed with mononucleosis. What explains the rash?
- A Allergic reaction to β-lactam
- B EBV-associated immune response
- C Cross-reactivity with cephalosporins
- D Drug-induced lupus
Correct answer: EBV-associated immune response
Ampicillin causes a characteristic rash in patients with EBV mononucleosis, not a true allergy.
Clavulanic acid is added to amoxicillin to achieve which effect?
- A Prolong half-life
- B Increase renal clearance
- C Improve CNS penetration
- D Inhibit β-lactamases
Correct answer: Inhibit β-lactamases
Clavulanic acid inhibits β-lactamases, protecting amoxicillin from enzymatic degradation.